1. Field of the Invention
This invention relates generally to offshore drilling vessels, and more particularly to a mooring unit for use on an offshore drilling vessel and to a method for retrofitting an a mooring unit.
2. Description of the Related Art
An offshore drilling vessel is essentially a massive, floating, mobile vessel used in the offshore exploration of oil and gas. In broad terms, the typical offshore drilling vessel generally includes two large pontoon hulls, at least four vertical support columns, and a drilling platform. The pontoon hulls float in the water horizontally and are parallel to one another. At least two support columns are attached to and extend vertically upwardly from each pontoon hull. The drilling platform is horizontally attached upon the vertical columns. The vessel is equipped with the necessary drilling tools to drill an oil and gas well into the ocean floor. The vessel is towed from drilling site to drilling site by one or more boats. When the vessel arrives at a drilling site, the vessel must be anchored to the ocean floor before drilling begins. Each corner of the vessel is anchored to the ocean floor by at least one large anchor. Each anchor is raised and lowered by its own mooring unit. In very broad terms, a mooring unit is essentially a giant hoist. It is quite common for each corner of the vessel to be anchored to the ocean floor by two large anchors. In this situation, two mooring units are mounted to each corner of the vessel; this is referred to as an "eight-point" mooring system. The mooring units are normally mounted below the vessel's drilling platform on shelves attached to the vertical columns.
There are three basic types of mooring units: (1) a mooring "winch," which uses large diameter wire rope to raise and lower the anchor; (2) a mooring "windlass," which uses large chain to raise and lower the anchor; and, (3) a "combination" mooring unit, which has both a winch and a windlass, and uses a combination wire rope/chain mooring line to raise and lower the anchor. With all three types of mooring units, the offshore drilling vessel is also equipped with a "fairlead" for each mooring unit. A fairlead is essentially a pulley. The fairleads are mounted to the vertical columns directly below each mooring unit. The wire or chain exits the winch or windlass and passes around its corresponding fairlead before being connected to the anchor. In broad terms, all three types of mooring units have a frame, at least one brake, at least one drive motor, at least one gearbox, and an electrical control system. A typical mooring winch also has at least one drum, to hold the wire rope, and a levelwind for each drum, with the levelwind guiding the wire rope back and forth across the drum as it is spooled out and wound up. An example of a commercially available mooring winch is Model No. ETW-350, which is manufactured by Skagit Products, of Houston, Tex. A typical mooring windlass, on the other hand, has at least one "wildcat," instead of a drum and levelwind. A "wildcat" is essentially a sprocket which grabs the chain link-by-link and either pulls it in or feeds it out. An example of a commercially available mooring windlass is Model No. WMD-48, also manufactured by Skagit Products. When retrieving an anchor, a windlass does not wind up the chain like a winch winds up wire rope. Instead, as the anchor is being retrieved, the chain, used with a windlass, passes over the wildcat and into a "chain locker" located inside the vertical column to which the windlass is attached; the chain simply piles up on the floor of the chain locker.
A combination mooring unit includes both a mooring winch and a mooring windlass. Thus, the mooring line with a combination mooring unit is a combination of wire rope and chain. The winch provides a large drum to store the required length of wire rope, while the windlass handles the chain portion of the mooring line. An example of a commercially available combination mooring unit is Model No. ETW-350/52, also manufactured by Skagit Products. A typical anchor chain used in a combination mooring line has an approximate diameter of three and one-quarter inches, is approximately 4,400 feet in length, and weighs approximately 100 pounds per foot. A typical wire rope used in a combination mooring line has an approximate diameter of three and one-half inches, is approximately 5,600 feet in length, and weighs approximately 23 pounds per foot. With a combination mooring unit, the anchor is always attached directly to the chain portion of the mooring line; the wire rope portion of the mooring line connects to the chain portion of the mooring line, not to the anchor.
When an offshore drilling vessel equipped with combination mooring units is moved from one drilling site to another, the anchors are held in a "stowed" position against brackets which are attached to the vertical columns below the fairleads. The anchor is held against the brackets by the chain, and the chain is held in place by the wildcat on the windlass. The remainder of the chain portion of the mooring line is stored in the chain locker. Further, during transport, the wire rope is stored on the drum of the mooring winch, and is disconnected from the chain portion of the mooring line. When the vessel arrives at the drilling site, the vessel must be anchored before drilling begins. To anchor the vessel, each windlass lowers its anchor away from its bracket and onto a boat. The boat then carries the anchor away from the vessel in the desired direction. The boat is advised to stop when most of the chain portion of the mooring line has been payed out. As known to those skilled in the art, when the chain has been payed out, a "tri-link" is used to connect the chain to the wire rope. At this point, the mooring line becomes a combination chain and wire rope mooring line, and the combination mooring line is controlled by the winch instead of the windlass. After the connection has been made, the boat is signaled to continue carrying the anchor to the desired location, thereby unwinding wire rope from the winch drum. When the boat reaches the desired location, the anchor is dropped to the ocean floor. After all anchors have been dropped in this manner, the drilling process may begin. When it is desired to move the drilling vessel to another drilling site, each anchor must be retrieved and secured to its corresponding vertical column as explained above. As explained below, it is during the process of retrieving the anchors with combination wire rope/chain mooring lines that the problem solved by the present invention arises.
At the beginning of the process of retrieving each anchor, in a typical situation, there is approximately 4,400 feet of anchor chain and approximately 5,600 feet of wire rope in the water. Thus, in this scenario, the total length of the combination mooring line in the water between the drilling vessel and each anchor is approximately 10,000 feet (or approximately 1.9 miles). A typical water depth in this scenario may be approximately 5,000 feet. One end of the chain is connected to the anchor and the other end is connected to one end of the wire rope. The other end of the wire rope is connected to the winch drum. At the beginning of the anchor retrieval process, the wire rope is at a layer very near the core of the drum. When the wire rope layer is very near the drum core, the winch is capable of pulling on the anchor with a much greater force ("winch pull force") than when the drum is full and the wire rope is at its outer layer. This is based upon the physical principle that torque is a product of force and distance. In this case, the torque delivered by the motor to the wire rope drum remains constant, and the distance from the drum core to the wire rope layer (i.e., the radius of the wire rope layer) varies. As the radius of the wire rope layer increases, the force must decrease since the torque remains constant. Thus, as the anchor is retrieved by the winch, and as the wire rope is evenly spooled by the levelwind onto the winch drum, the wire rope layer gradually moves away from the drum core, and the winch pull force gradually decreases. The existing combination mooring units presently in operation were designed with winches easily having the necessary winch pull force at the outer wire rope layer to haul the wire rope, chain and anchor up to a level high enough so that the chain could be connected directly to the windlass, and disconnected from the wire rope. However, as companies have extended their exploration efforts into deeper and deeper water, it has become necessary to utilize longer and longer lengths of chain and wire rope, thereby rendering inapplicable the original design parameters for the existing combination mooring units. Thus, the existing combination mooring units now in operation are being used under conditions more strenuous that those they were originally designed for. As will now be explained, a specific problem has arisen as a result of taking drilling vessels equipped with existing combination mooring units into greater water depths.
The specific problem occurs during the process of retrieving the anchors. More particularly, the problem occurs when the combination mooring line is still being reeled in by the winch. Stated differently, the problem occurs before the chain has been connected to the windlass and disconnected from the wire rope. As the winch is reeling in the anchor, and as the wire rope reaches the outer layer, where the winch pull force is at its lowest, the drive train component stress levels and deflections are highest. When in deeper water and operating parameters are more demanding than the original design parameters, the winch drive train component stress levels are further increased. If the retrieval loads continue to increase, the winch will simply stall. Thus, there is a need for a combination mooring unit having a winch with the capacity to raise the wire rope, chain, and anchor high enough to connect the chain directly to the windlass, and disconnect the chain from the wire rope, without exceeding the allowable design criteria, and without stalling. In addition, there is a need for an efficient and cost effective way to modify the existing combination mooring units presently in operation to meet these needs, instead of completely scrapping them and replacing them with new units. The present invention, as explained below, was developed in response to these needs.